1. Field of the invention
The present invention relates to a method and apparatus for automated measurement of piston ring tension. More particularly, the present invention relates to a method and apparatus for compressing a piston ring in a measuring fixture, with a thin high-strength band, and measuring the force exerted on the band, by the outwardly expanding ring. Even more particularly, the present invention relates to a method and apparatus of the type described, in which a portion of the fixture and piston ring are vertically reciprocated, in order to help remove any looseness from the band, caused by friction between the piston ring and the compression band, before measurement takes place.
2. Description of the Background Art
A manually operated method of measuring piston ring tension has been known for many years, in which a steel band is formed into a loop which is wrapped around a piston ring to be measured. The band is then placed under tension by pulling on a handle which is attached to one end of the band, the other end of the band being clamped in place. In this known design, the base of the handle is pivotally anchored to a support. A spring-actuated load gauge is connected to the steel band, between the handle and the loop which surrounds the piston ring, and the tension of the ring may be read directly off of the load gauge. This early method and apparatus suffered from problems with consistency and reproducibility.
Some later known devices exist for compressively measuring piston ring tension. Two such known devices are described in U.S. Pat. Nos. 3,946,602 and 4,249,415.
Certain aspects of this type of apparatus have become relatively standardized. Referring now to FIGS. 1-3, a steel band 10 is shown having a rectangular hole 12 formed therein. The band 10 also includes a narrowed diameter portion 14, and in order to form the band into a loop, a first end 15 of the band 10 is turned sideways, passed through the hole 12 in the band, and then straightened up as shown in FIG. 2. After passing the first end 15 of the band through the hole 12, the band 10 includes an integral loop 16 therein, with an adjustable size range which is determined by the length and spacing of the narrrowed diameter portion 14 in relation to the rectangular hole 12.
Referring now to FIG. 3, the band 10 is shown wrapped around a cylindrical fixture 17 having a horizontally oriented circumferential groove formed therearound, which receives a piston ring 18 therein. The fixture 17 is provided to hold and stabilize a piston ring 18 during testing thereof. The same fixture 17 is used over and over again, to test many different piston rings.
Often, a standardized master ring is used which exhibits a known and calibrated diameter. The master ring is used as a comparison device, to ensure that a piston ring measuring apparatus is set correctly. Where used, the master ring is checked first, and the machine readouts are adjusted, if necessary, to show the known tension thereof. Then, an unknown ring is tested.
A common problem has been encountered with this band type of device, in that when a fixture 17 and piston ring 18 is placed into a loop 16 of the band 10, and tension on the band 10 is increased, the band will not tighten down on the fixture and ring in a consistent and reprodudcible manner, unless the fixture and ring assembly is repeatedly tapped against a table top, or other work surface, as the band 10 is tightened therearound, to ensure that no looseness caused by friction remains in the loop 16. Where the band 10 is placed under tension without tapping the fixture and ring against a work surface, test results tend to vary with a single piston ring from test to test, instead of being reliable and reproducible. Accordingly, a step of tapping the fixture and ring assembly against a table or other support, as the band 10 tightens therearound, became a necessary part of the normal process of piston ring tension measurement.
Unfortunately, the tapping procedure of a given machine operator might be different from one day to the next, as well as different from the tapping procedure of another operator using the same machine, perhaps on a later work shift. Because of this above-described variability in test procedure, efforts began to be made to standardize the tapping operation as a part of piston ring tension measurement.
A recent design for an apparatus which may be used to measure piston ring tension is described in Japanese laid-open Patent number 6-19299, published Oct. 8, 1996, naming Hattori as the inventor. The disclosure of Japanese laid-open Patent number 6-19299 is herein incorporated by reference. In the apparatus of Hattori, a number of components are mounted on a rigid base, including a horizontally reciprocally movable first clamp coupled to an electronic load cell. The first clamp holds a first end of a steel band which is formed into a loop, and a horizontally reciprocally movable second clamp holds a second end of the steel band. The steel band encircles a piston ring to be measured. The piston ring rests on a carrying stand, which is separate from the base. The second clamp is attached to an electronic length measuring device, as well as to the band.
In order to make sure that looseness caused by friction in the band is removed in a consistent and reproducible fashion, before tension in the piston ring is measured, the apparatus of Hattori also includes a vertically reciprocally movable vibration plate, which is located in between the piston ring and the carrying stand. The vibration plate is free at a first end thereof, and is pivotally attached to the carrying stand at a second end thereof. An electric motor is horizontally mounted below the base, and the motor rotatably drives an eccentric cam, to alternately lift the free end of the vibration plate and drop it against the carrying stand. After any looseness caused by friction has been taken out of the band by reciprocal movement of the first and second clamps, acting in conjunction with the above-described movement of the vibration plate, the tension of the piston ring may be measured by the load cell, in conjunction with the electronic length measuring device.
Although the method and apparatus of Hattori is useful, and advanced the known state of the art at the time it was published, Hattori's use of an electric motor and eccentric cam to actuate the mechanical vibration plate is subject to wear and tear, and these components may fail after some time in service. It would be advantageous if a method and apparatus could be provided for measuring piston ring tension, which replaced the vibration plate, electric motor and eccentric cam of the Hattori apparatus with a more reliable means of vertically reciprocating a section of a piston ring, in a measuring device, which would not be as prone to mechanical breakdown.
A need still exists in the art for an improved method and apparatus for measuring piston ring tension, which does not have the shortcomings of the apparatus according to the design of Hattori.